In recent years, it has been increasingly demanded to suppress the consumption of fossil fuels in order to prevent global warming and protect the environment, which has affected various manufacturing industries. For example, automobiles, which are an indispensable part of transportation means in daily life and activities, are not an exception. There is a demand to improve fuel economy by, for example, reducing vehicle body weight. It is not allowed for automobiles, however, to simply reduce the vehicle body weight in terms of product functions. It is necessary to secure appropriate safety.
Many of the structural parts of an automobile are made of an iron-based material (in particular, a steel sheet). For reducing the vehicle body weight, it is important to reduce the weight of the steel sheet. Instead of simply reducing the weight of the steel sheet, which is not allowed as mentioned above, the weight reduction must be accompanied with securing the mechanical strength of the steel sheet. Such demand arises not only in the car manufacturing industry but also in various other manufacturing industries. Research and development efforts have been directed to a steel sheet that can have, due to improved mechanical strength, the same or a higher mechanical strength as compared with a conventional one even when the sheet is made thinner.
In general, a material having a high mechanical strength tends to become lower in shape fixability in shape formation work such as bending. It is difficult to form such material into a complicated shape. One of the solutions to the formability problem is what is called a “hot pressing method (also referred to as hot stamping, hot pressing, or die quenching)”. In the hot pressing method, a material to be formed is heated temporarily to a high temperature and the steel sheet softened by the heating is formed by pressing, and then the steel sheet is cooled. By using the hot pressing method, the material is once softened by heating to a high temperature, so that the material is easy to press. Furthermore, the mechanical strength of the material becomes higher due to a quenching effect during cooling after the shaping. Accordingly, the hot pressing method can provide a product having both a favorable shape fixability and a high mechanical strength.
When the hot pressing method is applied to a steel sheet, however, the surface of the steel sheet is oxidized to generate scales (compounds) due to heating the steel sheet to a high temperature of 800° C. or higher. Accordingly, a process to remove the scales (descaling process) is necessary after hot pressing, which deteriorates productivity. For the members and the like that require corrosion resistance, it is necessary to carry out anti-corrosion treatment and metal cover installation treatment on the surfaces of the members after the work process. A surface cleaning process and a surface treatment process are also necessary, which further deteriorates productivity.
As an example of a method for suppressing such deterioration in productivity, a covering layer can be installed on a steel sheet. In general, various materials including organic and inorganic materials are used for the covering layer on a steel sheet. Especially, galvanized steel sheets that have a sacrificial protection effect on steel sheets are widely used for steel sheets for automobiles and other products because the galvanized steel sheets provide anti-corrosion performance and suitability to steel sheet production technology. However, this may cause considerable deterioration in the surface properties because heating temperatures used in the hot pressing (700 to 1000° C.) are higher than the temperatures at which the organic materials decompose or the metal (e.g., zinc) melts or boils, so that the surface coating and the plating layer evaporate at the time of heating by hot pressing.
For this reason, it is desirable to use a steel sheet having an Al-based metal cover that has the boiling point higher than that of an organic material cover or a zinc-based metal cover (i.e., an Al-plated steel sheet) for the hot pressing method that involves high temperature heating.
The Al-based metal cover can prevent scales from being generated on the surface of the steel sheet, which leads to omitting a process such as descaling and improving the productivity of products. The Al-based metal cover also has an anti-corrosion effect and thus improves corrosion resistance. Patent Literature 1 below discloses a method in which hot pressing is performed on a steel sheet obtained by covering a steel sheet having a predetermined component composition with Al-based metal.
However, in the case where the Al-based metal cover is applied as in Patent Literature 1, the Al cover is melted and then an Al—Fe alloy layer is generated due to the dispersion of Fe from the steel sheet, depending on preheating conditions before hot pressing. Furthermore, the Al—Fe alloy layer grows until the Al—Fe alloy layer reaches the surface of the steel sheet in some cases. This Al—Fe alloy layer is so hard that there is a problem in that scratches are formed in the product by contacting with dies during the pressing work.
The Al—Fe alloy layer is not smooth on the surface and is inferior in lubricity. In addition, since the Al—Fe alloy layer is hard and tends to break, cracks are developed in a plating layer and powdering occurs, for example, resulting in lower formability. Moreover, flaked materials from the Al—Fe alloy layer and coming-off materials by strong abrasion on the surface of the Al—Fe alloy layer of the steel sheet attach on the dies. The Al—Fe intermetallic compound caused by the Al—Fe alloy layer then adheres to the dies, which leads to deterioration in the quality of products. To prevent this, it is necessary to periodically remove the Al—Fe intermetallic compound adhered to the dies, which is one of the causes for lowering the productivity of products and increasing the production cost.
Furthermore, the Al—Fe alloy layer is less reactive in phosphate treatment. Therefore, a chemical conversion coating (a phosphate coating), which is a treatment before electrodeposition coating, cannot be generated on the surface of the Al—Fe alloy layer. Even when a chemical conversion coating is not generated, corrosion resistance after paint coating would be favorable if adhesion with paint is favorable and Al deposition amount is large enough. However, an increase in Al deposition amount leads to a larger amount of the Al—Fe intermetallic compound adhered to the dies.
The adhesion of the Al—Fe intermetallic compound includes the case where the flaked materials from the Al—Fe alloy layer attach on the dies and the case where the coming-off materials by strong abrasion on the surface of the Al—Fe alloy layer attach on the dies. In performing hot pressing on a steel sheet having a surface coating, an increase in lubricity makes an improvement for the case where coming-off materials by strong abrasion on the surface of the Al—Fe alloy layer attach on the dies, but this increase in lubricity is not effective for an improvement for the case where flaked materials from the Al—Fe alloy layer attach on the dies. To alleviate the attachment of the flaked materials from the Al—Fe alloy layer on the dies, it is most effective to lower the Al deposition amount in the Al plating. However, lowering the Al deposition amount causes deterioration in the corrosion resistance.
Hence, Patent Literature 2 below discloses a steel sheet with an objective to prevent scratches from occurring in a product during work. In the steel sheet disclosed in Patent Literature 2, an Al-based metal cover is formed on the surface of a steel sheet having a predetermined component composition, and, on the surface of the Al-based metal cover, there is formed a coating made of an inorganic compound containing at least one of Si, Zr, Ti, and P, an organic compound, or a complex compound thereof. For the steel sheet with a surface coating formed thereon as disclosed in Patent Literature 2, the surface coating does not come off during pressing after heating, so that the surface coating can prevent scratches from being formed during pressing.
Patent Literature 3 below discloses a method in which a wurtzite compound, particularly zinc oxide, is formed on the surface of an Al-plated steel sheet. The method disclosed in Patent Literature 3 is a method that improves lubricity in hot state and chemical conversion treatability, and adds a binder component to a surface coating in order to secure the adhesion of the surface coating before hot pressing.